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Calcium silicate hydrate (C‐S‐H) synthesis follows multistep nucleation processes ranging from amorphous globules to nanocrystals. This type of nonclassical nucleation pathway attracts extensive interest and yet related molecular processes are mostly unexplored. Herein, atomic‐level structure evolution during this process is unveiled. From inside to outside of the globule, the atomic arrangement evolves from amorphous configuration to layered stacking of calcium silicate layers with layers spaced by 1.1 nm. Numerous cations surrounding the globules provide extra calcium ions to form nano C‐S‐H foils at the edge of the globule. The polymerization to form calcium silicate chains has almost been completed before 5 min reaction, and the C‐S‐H structure formation is basically based on the self‐assembly to be calcium silicate layers and then lamellar crystal inside the globule. Silicate chain defects inhibit a‐ and c‐axes crystallization, but promote b‐axes one, leading to anisotropic growth of crystalline C‐S‐H. The defects lead to the crystallization pathways conversion from Si–O–Si bonds formation to Ca2+ electrostatic linking. [ABSTRACT FROM AUTHOR]

The mechanical properties of organic/inorganic composites at a macroscopic level are largely determined by the interaction mechanisms at the interface. However, there have been limited microscopic studies on these interactions. To address this knowledge gap, this study used molecular dynamics (MD) simulation to examine the interfacial structures, kinetics, and energetics between calcium silicate hydrate (CSH) and polymers. The purpose of this investigation is to shed light on the factors contributing to the variations in mechanical properties of the materials and to provide atomic-level guidance for nanocomposite toughening studies. Three polymers, polyacrylamide (PAM), sodium polyacrylate (PAAS), and polymethacrylic acid sodium sulfonate (PAMAS), were incorporated into the nanochannels of CSH sheets to create polymer/calcium silicate hydrated composites. Our simulations revealed that calcium atoms at the CSH surface act as intermediaries bridging polymers and the CSH through Ob─ Casur─ Op and metal atoms in polymer functional groups through Ob─ M─ Op. Furthermore, hydrogen bonds between the interface water molecules and the polymer and CSH matrix were observed through Op─ Casur─ H2O─ Ob and Op─ M─ H2O─ Ob bonds. Uniaxial tensile simulations were carried out to assess the mechanical behavior of composites, with results indicating that all three materials failed at their interfaces. Analysis of chemical bonding at the point of failure revealed that PAM/CSH exhibits the highest number and stability of chemical bonds and thus the best mechanical properties, followed by PAAS/CSH, with the worst being PAMAS/CSH. Our study provides fundamental atomic-level insights into the differences in interaction mechanisms and macroscopic mechanical properties of composites through molecular dynamics simulation, offering a theoretical basis for polymer modification of CSH and the genetic improvement of cementitious materials. [ABSTRACT FROM AUTHOR]

The composition and properties of the interface are the key points which determine the properties of fiber-reinforced cement–based composites. The bonding performance between fibers and different hydration products at the interface is the basis for the understanding of the interface transition zone. In this paper, the influence of different hydration products on interfacial bonding performance was investigated using the molecular dynamics method. The three models were established to study the difference in the static structure of the interface between different hydration products and polyvinyl alcohol (PVA) fibers, and then the influence of different hydration products on the interfacial adhesion properties was analyzed using fiber pull-out simulation. The results showed that the interfacial adhesion properties between PVA fiber and calcium silicate hydrate (PVA–CSH) was the best. The PVA–CSH interface is connected mainly by Ca-O bonds and H bonds, and the stability of the chemical bonds is very high. The strong interface interaction gives the CSH–PVA interface good adhesion performance, so it is necessary to overcome a large interfacial adhesion force when the fiber is pulled out. The PVA–AFt (ettringite) interface also is connected by Ca-O bonds and H-bonds. However, the stability and coordination number of the chemical bond are lower than those of the CSH–PVA interface. There are only a few weak H-bonds at the PVA–calcium hydroxide (CH) interface, which resulted in the worst interfacial adhesion performance; the force required to pull fibers out of the matrix was very small. [ABSTRACT FROM AUTHOR]

Although amino trimethylene phosphoric acid (ATMP) has been widely used as a retarder for Portland cement, its effect on cement hydration is poorly understood at the atomic level. In this study, we combine static calculation and ab initio molecular dynamics (AIMD) simulation to reveal the mechanism of the effect of ATMP on the initial stage of C3S hydration from multiple perspectives, quantitatively analyze the structural reconstruction and charge migration at the ATMP/C3S interface in the aqueous environment. By adsorbing on the surface of C3S, ATMP occupies the adsorption site of water molecules. Compared with the pure C3S surface, the addition of ATMP delays the hydroxylation of the C3S surface and inhibits the formation of Ca‐Ow bonds. This work gives new insights into understanding the hydration of C3S with ATMP and offers new approach of designing new cement retarder at the molecular level. [ABSTRACT FROM AUTHOR]

Calcium silicate hydrate (CSH) plays a crucial role in concrete by controlling its properties and durability. The degradation of CSH often signifies concrete damage. Polydimethylsiloxane (PDMS) is commonly used to protect concrete from sulfate corrosion; however, the comprehensive mechanistic understanding of its protective effects against CSH remains limited. Here, molecular dynamics (MD) simulations were employed to explore atomic-scale interactions between PDMS coatings and CSH in a sulfate-rich environment. Our results reveal that PDMS mitigates sulfate-induced CSH decalcification by forming a positively charged layer, ultimately reducing sulfate bonding by 83.3% compared to the blank group. Molecular structure analysis highlights key hydrogen bonding and calcium–oxygen bonding interactions that are critical for this protection. Higher polymerization stabilizes substrate adsorption, reducing surface diffusion to 33.3% of low-polymerization PDMS, thereby enhancing protection. Additionally, water molecule interactions with the CSH matrix are negatively correlated with the amount of adsorbed sulfate. Simulation results offer valuable insights into the molecular-level dynamic response of the material, contributing to a deeper understanding of the protective mechanisms of PDMS against sulfate-induced CSH degradation in concrete. These findings can guide experimenters and engineers in designing more effective protective coatings for concrete exposed to sulfate-rich environments, thereby laying a foundation for further experimental research and the development of concrete materials with enhanced durability under challenging environmental conditions. [ABSTRACT FROM AUTHOR]

The application of polysiloxane coatings represents an effective means of improving the durability of cementitious materials. However, conventional polysiloxane coatings with a single component are unable to provide adequate surface protection for cementitious materials due to the disordered nature of their molecular structure. In this paper, four functional monomers are employed to regulate the molecular structure of octyl polysiloxane, and polysiloxane composite emulsions with two-components were prepared in an innovative manner. The results demonstrated that the two-component polysiloxanes induced the formation of a plentiful of silicate crystals with low Ca/Si ratios on the surface of the hydration products, which reduced the proportion of polyhazardous pores to 4.56 %. The composite polysiloxane coating enhances the crystallinity of the cementitious material by repairing crystal defects and improving the surface mechanical characteristics of the cement substrates. The static capillary water absorption of the TOS coating modified by TEOS was reduced by 30.55 %, and the chloride and sulphate corrosion were reduced by 31.96 % and 27.56 %, respectively, and the carbonization depth was reduced by 46.7 %, in comparison to the conventional silane coating. The TOS coating enabled the concrete to achieve a relative dynamic modulus of 88.1 % after 100 freeze-thaw cycles. Moreover, the composite polysiloxane coating effectively resists acid-alkali corrosion and ultraviolet aging on the microstructure of the cementitious materials, thereby maintaining the superior impermeability of the cement substrates. • Four functional monomers were used to regulate the molecular structure of octyl polysiloxanes. • The two-component polysiloxane networks have a greater degree of crosslinking and branching. • The two-component polysiloxane reduced the proportion of polyhazardous pores within the cementitious material to 4.56 %. • The two-component polysiloxane coatings offer superior resistance to ionic attack, carbonation and freeze-thaw deterioration. • The two-component polysiloxane coatings provide stable surface protection against acid - alkali corrosion and UV aging. [ABSTRACT FROM AUTHOR]

The bonding performance between epoxy resin and cement-based materials affects the load transfer capacity between fiber-reinforced polymers (FRP) and concrete. FRP-reinforced concrete structures are inevitably exposed to aggressive environments during service, triggering interfacial deterioration and premature structural failure. To assess the stress state of FRP-reinforced concrete structures and to predict their long-term durability, a fundamental investigation of the bonding performance of the epoxy/concrete interface in aggressive environments is required. In this study, the bonding performance between epoxy resin and calcium silicate hydrate (CSH) in water and salt environments is investigated using molecular dynamics (MD) simulations. The adhesion energy of epoxy/CSH interface calculated via combine MD simulation and Bell's model theory is dramatically reduced in water and salt environments. Analysis of the static structures and dynamic properties reveals that the adhesion between the epoxy resin and CSH matrix is mainly dependent on the O–Ca–O and H-bond connections, while the water and salt environments significantly weaken these connections and disrupt the integrity of the epoxy/CSH interface. This study provides a molecular-level understanding of the bonding degradation at the epoxy/CSH interface in aggressive environments. This study provides a molecular level understanding of the bonding degradation between epoxy and CSH matrix in aggressive environments. [ABSTRACT FROM AUTHOR]

Graphical abstract The morphology of the C-S-H gel with different indentation modulus and the packing C-S-H molecules. Highlights • The indentation modulus of C-S-H gel ranges from 25 to 35.9 GPa. • Water content and packing morphology influence modulus of C-S-H gel. • The stiffness of the C-S-H gel is weakened with increasing Ca/Si ratio. • The C-S-H gel shows anisotropic mechanical properties at nano-scale. • The gel strength is weakened by the silicate defection and nanoporosity. Abstract Calcium silicate hydrate (C-S-H) is the most important hydration product in cementitious material. Due to its complexity, the investigation on its mechanical properties is still in progress. In this study, synthesized C 3 S was utilized to produce relatively pure C-S-H gels with two different water/solid ratios. Then mechanical properties of the C-S-H were measured and characterized by nano-indentation and SEM/EDX/BSE. Two types of C-S-H with Ca/Si ratio around 1.8 were observed through statistical analysis of the indentation modulus ( E r) and hardness ( H ). The looser one possessing lower Er around 25–30 GPa while the denser one possesses higher Er of 35.9 GPa. Both phases have higher stiffness than those of high density ( HD ) and low density ( LD ) phase obtained from cement pastes. Furthermore, molecular dynamics provides mechanical understanding of C-S-H gel at nano-scale. Based on the crystal structure of tobermorite 11 Å, 64 models with Ca/Si ratios from 1.3 to 2.0 were constructed for uniaxial tension test. The stiffness and cohesive strength of the C-S-H gel are greatly influenced by Ca/Si ratio, water content and nano-porosity. With increasing Ca/Si ratio from 1.3 to 2.0, long silicate chains were broken to shorter ones and more water molecules penetrated into defect regions. The destroyed silicate skeleton and the "hydrolytic weakening" effect resulted in the reduction of the mechanical properties of C-S-H gels. Concurrently considering the intrinsic porosity and chemical compositions, the appropriate micromechanical models are utilized to upscale the molecular properties to higher level. [ABSTRACT FROM AUTHOR]

Molecular dynamics was utilized to investigate ionic structure, dynamics and adsorption behaviors of the sulfate and sodium ions highly solvated in the nanometer channel of calcium silicate hydrate (C–S–H) with two different Ca/Si ratios. Due to H-bond connection from silicate oxygen sites and attraction of surface calcium ions, a stable adsorption layer of water molecules was formed in the vicinity of the hydrophilic silicate channel: clear layered arrangements, strong orientation preferences and extremely low diffusion coefficients. Furthermore, the adsorption mechanisms differ for the sulfate and sodium ions confined in the gel pore of C–S–H: while sodium ions are connected onto the non-bridging oxygen sites by Na-O nb bonding, the sulfate ions can be attracted by the calcium ions, leading to the aggregation of Ca-SO 4 ionic pairs. In the surface region, since the hydration shell of the solvated ions are partly substituted by the adsorbed water molecules, surface calcium ions and silicate oxygen sites, the diffusion of these ions is dramatically immobilized. Both the adsorbed Na + and SO 4 2− ions display extended exchange time for their solvation shells and longer residence time for their ionic clusters, as compared with those in aqueous solution. Besides, the increase in the surface Ca/Si ratio disturbs the local ordered arrangements and reduces the stability of the hydration layer. At high Ca/Si ratio, the highly solvated calcium ions contribute to the transformation from the outer adsorbed sulfate ions to the inner adsorbed species, enhancing the anions immobilization near the silicate surface. [ABSTRACT FROM AUTHOR]

The hydration and dissolution of tri-calcium silicate mineral is of great importance for guiding the material design. In order to understand the hydration mechanism of C 3 S mineral in sodium sulfate solution, molecular dynamics was employed to construct three interface models of C 3 S and Na 2 SO 4 solution of 0 mol/L, 0.79 mol/L and 1.58 mol/L and the structure, dynamics and dissolution properties of the interfacial species were investigated. Due to hydrophilic nature of C 3 S surface, the water molecule in hydrate layer deeply tessellated on the vacancy surface region shows high intensity packing, ordered arrangement, enlargement of the dipolar moment value and orientation preference. With progressively increasing simulation time, the hydroxyl groups and calcium atoms in the high reactive surface gradually escape from the substrate bonding restriction and diffuse away to the solution region. Furthermore, both ions prefer to adsorbing on the surface of tri-calcium silicate mineral: the highly solvated surface calcium ions associate with the oxygen sites in the sulfate ions, forming the Ca SO 4 ionic pairs in the interfacial region; the sodium ions are attracted by the non-bridging oxygen in the silicate monomers, forming Na O connection. The surface adsorbed water molecules, sulfate and sodium ions, strongly associated with the calcium atoms and hydroxyl groups, form a ionic and hydrate layer, stabilizing the tri-calcium silicate surface and inhibiting the dissolution of ions in C 3 S. With increasing ions’ concentration, the sulfate and sodium ions in the solution are more probable to form cation-anion pairs and further accumulate to the ionic cluster. [ABSTRACT FROM AUTHOR]

[Display omitted] • The addition of PVA cause the interfacial intertwined network. • PVA presents a strengthening mechanism for the interface ion-pair. • The load transfer is partitioned and transferred in multiple paths by PVAs. The surface modification of carbon nanotubes (CNT) can further improve the ductility of the cement/CNT composites. Atomic simulations were employed to decode the alteration of the interfacial load transfer mechanism by PVA addition, non-covalent bond modification of CNTs. The results of the pull-out simulations show that PVA significantly enhances the load transfer at the organic–inorganic interface. Interfacial load transfer depends on effective interfacial interactions, which in turn depend on interfacial chemical bonding. The interfacial region is more complexly reorganized by PVA, accompanied by the renewal of interfacial bonding. The addition of PVA could add new ion pairs (O CNT -Ca-O PVA -Ca-O CSH and H CNT -O PVA -Ca-O CSH) and lots of hydrogen bonds without detriment to the original linkage (O CNT -Ca-O CSH), in turn forming a strong network of chemical bonds intertwined with direct and indirect connections. Due to the intertwined network, the load transfer at the interface is partitioned and transferred in multiple paths, contributing to an increased interfacial strength. During the pull-out process, the load distribution between CNT, PVA and CSH evolves constantly, which is beneficial to damage retardation of the cement/CNT composites. [ABSTRACT FROM AUTHOR]

As the most important binding phase of cement paste, calcium silicate hydrate can determine the mechanical properties of cementitious materials. Structural, dynamic and mechanical properties of calcium silicate hydrate gel at the colloid level (∼15 nm) were investigated for the first time in light of molecular dynamics. Calcium silicate hydrate models were constructed by aggregation of colloids consisting of layered molecular structures. Five calcium silicate hydrate samples were first constructed by 108 spherical calcium silicate hydrate particles coated by water molecules of thickness from 0 nm to 0·2 nm. The built models including 200 000∼300 000 atoms were subsequently subjected to uniaxial tension testing to determine the mechanical properties. Structurally, water molecules coating the colloids disjoin the interaction between neighbouring calcium silicate hydrate particles by replacing the stable ionic-covalent bonds with low-strength H bonds. Water molecules in the interparticle region diffuse faster than structural water molecules inside calcium silicate hydrate gel, which reduces the stability of colloid aggregation. With increasing water-coated molecule layers, both the stiffness and cohesive force of calcium silicate hydrate gels are significantly weakened. During the tensile process, the region rich with water-coated molecules become the crack propagation channel, which accelerates the tensile failure of calcium silicate hydrate gel in the saturated state. [ABSTRACT FROM AUTHOR]

Calcium silicate composite (CaO)x(SiO2)1-x has significant applications in the bioactive materials in medical treatment and cementitious materials in construction engineering. In this study, to unravel the role of calcium atoms on the silicate composite, the molecular dynamics (MD) technique was used to simulate the structures, dynamics, and mechanical properties of (CaO) x (SiO2)1-x systems, with x varying from 0 to 0.6. The Feuston--Garofalini model was employed to describe the interatomic interactions in the systems. Q species, the connectivity factor, shows that the increase in calcium content in the silicate composite can lead to the depolymerization of the silicate network. Due to the high diffusion rate, the presence of Ca atoms also weakens the stability of the chemical bonds in the system. With the increasing calcium content, the molecular structure of the silicate skeleton is transformed from an integrity network to separated short chains, which significantly decreases the stiffness and cohesive force of the calcium silicate composites. On the other hand, the uniaxial tension response of the calcium silicate composites suggests that at the postfailure stage, Ca atoms associate with the nonbridging oxygen atoms and the reconstructed Ca-O connection slows down the irreversible damage of the composite, hereby enhancing the plasticity. [ABSTRACT FROM AUTHOR]

Due to its complexity at nanoscale, calcium silicate hydrate (C-S-H), the dominant binding phase in cement hydrates, is not yet completely understood. In this study, molecular dynamics was employed to simulate the hydration products at low and high calcium/silicon ratios. It was found that two morphologies of calcium silicate hydrate gels can be distinguished - a branched structure at low calcium/silicon ratios and an ellipsoid particle structure at high calcium/silicon ratios. Using virtual X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS) techniques, the simulated structures were characterised, confirming that they show features of calcium silicate hydrate as revealed by experimental approaches. The short-range structures of calcium and silicon atoms and the distorted calcium tetrahedrons resemble the features of silicate glasses obtained from experiments, implying the amorphous nature of the local structure in calcium silicate hydrate gel. Furthermore, formation mechanisms for the two morphologies are proposed. In the hydration process, calcium ions play roles in depolymerising the silicate structure and preventing the amorphous network formation. Therefore, at low calcium/silicon ratios, the reaction is governed by silicate skeleton growth, but at high calcium/silicon ratio, aggregations of calcium ions and short silicate chains dominate. [ABSTRACT FROM AUTHOR]

[Display omitted] • Proposed the atypical adsorption phenomenon of PCE superplasticizers on the C-S-H surface. • Reveal the aqueous environment to weaken the interfacial bonding from the hydrogen bond to the van der Waals interaction. • Establish the relationship between electronic bonding and the adsorption conformation through free energy. Although PCE adsorption occurrence is widely perceived to correlate to the anchoring of the carboxyl group on the PCE backbone, the understanding of the details is still insufficient. Here, we investigate the microstructural behavior of PCE adsorption with and without aqueous environments by molecular dynamics simulation and semi-empirical quantum chemistry calculation. We found that the introduction of an aqueous environment can change the mode of adsorption from strictly anchoring to atypical that the chains of PCE can sweep around the C-S-H surface. Two kinds of segments on the C-S-H surface have anchored the PCE, including Ca ions and hydroxyl. The aqueous environment alters the domination segments from Ca ions to hydroxyl. Further electronic analysis proves the weakening of bonding strength between PCE and C-S-H by solvation effects. This weakened interaction promotes the potential well of free energy away from the C-S-H surface and finally caused the atypical adsorption of PCE. [ABSTRACT FROM AUTHOR]

[Display omitted] • The relations of tensile strength and component of C–S–H/PAM are clarified. • Hydrolytic reaction can transform the weak H-bonds. • The calcium atoms play the essential role for connecting C–S–H and PAM. Hydrolytic reaction of polymers influences the interfacial bonding of cement-polymer composites. The interfacial deterioration mechanism of the calcium silicate hydrates (C–S–H) incorporated by multi-layered Polyacrylamides (PAM) is revealed by molecular dynamics (MD) simulation. To consider the influence of hydrolysis reaction, the amide groups (CONH 2) of PAM is transformed to deprotonated carboxyl groups (–COO−) with the substituted ratio of 100% (fully hydrolysis (FH)), 75% (higher hydrolysis degree (HH)) and 50% (lower hydrolysis degree (LH)). Simulation results reveal that the interfacial chemical bonds are mainly attributed to Ca-O bonds, that is the calcium atoms play a predominated role in connecting with the silicate chains and oxygen-containing functional groups in PAM. Also, in the interior region of multi-layers polymers, hydrolytic reaction can transform the weak H-bonds connection to stable ionic bond connection, which strengthens the inter-polymer network at a high hydrolytic degree. Furthermore, uniaxial tensile modeling results exhibit that the failure mode is greatly dependent on hydrolytic degree for C–S–H/PAM composites. The hydrolytic reaction of PAM not only strengthens interfacial strength but also enhance the ductility of C–S–H/PAM composites. While at a low hydrolytic degree, the composite is stretched to fracture in the interior region of PAM polymers, failure happens at the interface of C–S–H and PAM at a high hydrolytic state. The properties of C–S–H/PAM composites decoded at the atomic scale level might guide the organic–inorganic composites with enhanced mechanical properties and durability. [ABSTRACT FROM AUTHOR]

• Effect of SCA modification on NaCl solution in the C-S-H gel and rubber nanopore. • The hydrophobicity of rubber and the duality of silane are confirmed. • Rubber and silane are not conducive to the adsorption of Na+ and Cl-. The corrosive ions in the coastal environment invade the rubber concrete structure through capillary adsorption, which greatly limits the reuse of waste rubber. Silane coupling agent (SCA) modified rubber concrete gradually enters people's field of vision due to the improvement of its mechanical properties, and at the same time, it helps to improve the durability of the material. However, the interaction mechanism of action of ions with SCA and rubber is still unclear. Based on molecular dynamics theory, this study simulates the NaCl solution in the pores of C-S-H gel and rubber before and after SCA modification. Through the analysis of the local structure and dynamic performance of water molecules and ions, it is found that the direction of the distribution of water molecules at the rubber interface is opposite to that of C-S-H. The order of water molecules on the silane interface is enhanced, and it has a richer hydrogen bond network. The hydrophobicity of rubber has been confirmed on a microscopic scale, while silane has a duality to water molecules. The silanol group close to the C-S-H substrate is hydrophilic and can form hydrogen bonds with water molecules, while the tail alkane group is highly hydrophobic. Moreover, rubber and silane are not conducive to the adsorption of Na+ and Cl- and will change the coordination of ions at the interface. Although there are many kinds of hydrogen bonds between silane and water molecules, the stability is poor. The addition of silane will slow down the movement speed of water molecules in the pores, thereby indirectly improving the resistance of the SCA modified rubber cement-based material to corrosion by chloride ions. [ABSTRACT FROM AUTHOR]

• A new strategy is proposed to simulate the capillary adsorption in the C-S-H nanometer channel with epoxy coating. • The presence of epoxy molecules significantly retards the migration of water and detrimental ions in the C-S-H gel channel. • The water resistance and adhesion weakening effect are explained from molecular level. The migration of water and ions in the interfacial region of cement hydrate and epoxy determines the durability of FRP reinforced concrete composite. In this study, to better understand the nanoscale mechanism of the composite material subject to chemical attack, molecular dynamics is utilized to investigate capillary adsorption of NaCl and Na 2 SO 4 solution in the nanometer channel of the calcium silicate hydrate (C-S-H) coated with epoxy molecules. The calcium ions immobilize the epoxy stably on the interior surface of C-S-H gel by connecting with silicate tetrahedron in the C-S-H and the oxygen-contained functional group in the epoxy molecule. It explains the good adhesion source between epoxy resin coating and cement-based material from the molecular perspective. The presence of epoxy molecules, narrowing the gel pore entrance and increasing the hydrophobic properties of the interior surface, significantly retards the migration of water in the C-S-H gel channel. Additionally, the silicate chains in the C-S-H gel surface provide non-bridging oxygen sites to attract the Na ions by forming Na-O bond. The SO 4 and Cl ions can associate with the Na and Ca ions to form the anion-cation pairs. The interfacial chemical bond and ionic pair formation in the solution further reduce the transport rate of the ions. On the other hand, the ions and water in the gel pore can gradually ingress into the interfacial region between the epoxy molecule and the C-S-H substrate, which results in the Ca ions diffusing away from the substrate, part of epoxy chains dissociating from the silicate chains and finally weakening the interfacial connection. The distorted epoxy resin adsorbed on the C-S-H gel substrate acts like a polydispersity brush, changing the flow rate of water molecules at the interface region of the channel. The water resistance mechanism of epoxy coating and weakening effect from the detrimental ions can provide molecular insights for the durability study on the FRP reinforced concrete composite. [ABSTRACT FROM AUTHOR]

Cementitious materials in durability can be improved by membrane protection and osmotic hydrophobicity with Polystyrene acrylate (PSA) and polysiloxane (PSE), respectively. However, due to the significant differences in molecular structure, the fabrication of PSA- PSE composite coatings with multiple protective effects have been a thorny issue. In order to enhance the interfacial binding effect between PSA and PSE, and to solve the problem of poor stability caused by poor compatibility, this paper innovatively utilizes the topological network structure growth regulation mechanism of secondary surface functional nanoparticles to prepare PSA-PSE copolymer emulsions with highly crosslinked star and dendrimer structures, respectively, with branch grafting ratios of over 94% and semi-crystallinity of over 5.39%. As a consequence of surface treatment, numerous dense calcium-silicate crystals with low Ca/Si ratios are deposited inside the cementitious materials, decreasing the volume proportion of harmful micropores and connected capillary pores to below 10.4%. The modified copolymer coatings decrease the capillary water absorption and static permeability of concrete by over 55%, and lead to nearly 80% reduction in carbonation depth of cement hydrates, and have an adhesion strength of up to 3.3 MPa. Particularly, the proportion of ordered hydrogen bond within the composite coating reached 45.1%, demonstrating excellent reversible elastic deformation recovery performance. [Display omitted] • Multicomponent PSA-polysiloxane composite emulsions with gradient topology were designed and synthesized. • The copolymer coatings decrease the proportion of harmful connective pores to below 10.4%. • The modified copolymer coatings exhibit superior water repellency, ionic resistance and carbonation resistance. • The modified copolymer coatings have superior tensile performances and adhesion strengths of 3.3 MPa. • The modified copolymer coatings exhibit superior reversible elastic deformation recovery performances. [ABSTRACT FROM AUTHOR]

In this study, molecular dynamics is utilized to investigate on the migration of NaCl solution in the nanochannel of calcium silicate hydrate under external electric field ranging from 0 to 0.05 V/Å. The generated ionic current and flux are linearly related with the electric field intensity. While the directional movement of Cl− ions accounts for more than 85% of the ion currents, the migration of Na+ ions and Ca2+ contributes little percentage to the ionic current. The little contribution from cations is attributed to that the silicate chains near the C–S–H surface provide plenty of the oxygen sites to associate with the cations by forming Na–O s and Ca–O s bond, immobilizing the migration of the cations. However, the external electric field can weaken the bond stability of the Na-O s and Ca-O s bond, which can be proved from the hydration time of the ion-water cluster and the resident time of ions in the silicate chains. Furthermore, the ions accelerating effect induced by the external field is dramatically retarded by the nanoconfinement. The diffusion coefficient of ions ultra-confined in 3.4 nm and 5.6 nm pores is 3 magnitudes lower than the mobility of ions in the bulk solution. Due to the different H-bond and ionic bond restriction from the C–S–H substrate, the mobility of Na, Cl and Ow ranks in the following order: Cl > Ow > Na, unlike the same transport rate of solution species in the bulk solution. The results obtained from this work may help clarify the diffusion mechanism of ions in concrete structures during the electrochemical desalination processes and is helpful for the design to improve durability of materials. [ABSTRACT FROM AUTHOR]

Graphical abstract The Na and Cl ions are filtered from the solution by the C-S-H gel nanopores. Highlights • Water transport in the gel pore follows the classic LW capillary theory. • C-S-H gel pore immobilizes the Na and Cl ions by interfacial interaction. • Na ions adsorb on the silicate chain in C-S-H gel by Na-O bond. • Cl ions can form ionic pair with Ca ions on C-S-H surface. • The ions transport rate is slower than that of water in small nanopores. Abstract The movement of water and ions in the calcium silicate hydrate (C-S-H) gel pores determines the durability for the material. In this study, molecular dynamics was utilized to study aqueous NaCl solution capillary transport through the C-S-H gel pore with pore size of 3.5 nm, 2.5 nm, 1.5 nm and 1 nm. The penetration depth for the solution advancing frontier with menisci shape follows a parabolic relation as the function of time, matching well with classic LW capillary adsorption theory. The progressively reduction for the solution/C-S-H contact angle reflects the hydrophilic nature of the C-S-H surface. With decreasing of C-S-H gel pore size, the ions are filtered by the small C-S-H gel pore, with water invading deeply in the gel pore, and chloride and sodium ions remaining in the region of gel pore channel entry. The ultra-slow transport mechanism for ions in the nanometer channel has been further explained by the local structure and dynamics of ions hydrated ultra-confined in the gel pore. While the silicate chains in C-S-H surface can provide non-bridging oxygen sites to associate with the sodium ions by Na-O s connection, the courter calcium ions in C-S-H surface can capture the chloride ions, forming the ionic pair. Both Na-O s and Ca-Cl pairs have longer resident time that the unstable H-bond connection between water and solid oxygen. Furthermore, the transition region with slow mobility is formed at the channel entry region due to the coupling immobilization effect by both surfaces normal and parallel to the transport pathway. The presence of transition zones results in the necking of channel entry, which inhibits the ions with larger hydration shell from penetration. Additionally, at channel with size less than 2 nm, the Ca-Cl and Na-Cl ionic pairs accumulate to cluster and are highly concentrated in the center of the gel pore, blocking transport pathway for water molecules and ions. [ABSTRACT FROM AUTHOR]

Hydration of minerals, playing fundamental role in many technological applications, involves multi-stage chemical reactions, morphology changes and compositional variations. For the first time, the hydration mechanism for the tricalcium silicate was investigated by the combination of C 3 S hydration experimentally and polymerization of silicate monomers via molecular dynamics simulation. Tricalcium silicate was self-synthesized and utilized to hydrate with water at the water/solid ratio of 50. Samples hydrated from one day to one year were obtained and characterized by XRD, IR, SEM, NMR and TEM techniques. Based on SEM observation, the morphology of hydrated C 3 S transformed from small localized to foil-like wrinkled clusters with fibrillar-like wrinkles distributed randomly. Wrinkled product of C-S-H possessed poor-crystallined or amorphous feature based on XRD and TEM analysis. Such was also evidenced by the fact that no layered crystal phase was observed in simulated calcium silicate gel clusters. Besides, the simulated pair distribution function of C-S-H gel demonstrating the structure feature of short range order and long range disorder, also confirmed glassy structural nature for the C-S-H gel. Furthermore, based on NMR and SEM results C/S ratio changed to 1.2 and the mean silicate chain length in C-S-H was obtained at around 4 after hydration for one year with Q1 and Q2 species representing linear silicate chain structure present dominantly. The molecular dynamics helped illuminate the hydration mechanism that the morphology of C-S-H gel was closely related with C/S ratio. At C/S ratio of 1, solid branched 3-dimmensional cluster was segregated from the solution while ellipsoid colloid particle with the size of 4 nm × 4 nm × 8 nm was formed at C/S ratio of 1.75. It was also found that calcium ions prevented the polymerization reaction of silicate monomers and silicate chains were shortened significantly at high Ca/Si ratio, amplifying the predominate percentage of Q1 species in the C-S-H gel. [ABSTRACT FROM AUTHOR]

Silane coupling agents (SCAs) are utilized to enhance the impermeability of concrete surfaces, whose adhesion force is affected by the interfacial interactions in between that remain unclear. In this study, we probe the adsorption tendencies of ethyltriethoxysilane (ETES) onto C-S-H surfaces by employing density functional theory (DFT) calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations. Results suggest that significant chemisorption occurs at the CaOH (1.5) , SiOH (1.0) , Ca (1.0) sites C-S-H surfaces. Electronic structures analysis proved that the chemisorption of hydroxyl sites arises from the overlap of H and O atomic orbitals, resulting in the formation of hydrogen bonds. The chemisorption of the Ca (1.0) site is caused by electrostatic adsorption between calcium and oxygen. Adsorption energy results show: E CaOH(1.5) > E Ca(1.0) > E SiOH(1.0). Moreover, ETES remains transverse adsorption on low Ca/Si C-S-H surfaces, while displaying longitudinal adsorption on high. Interaction analysis illustrates the configuration discrepancy arising from the absence of bridge silicon-oxygen tetrahedrons which possess vdW interactions with ethyl. Notably, transverse adsorption covers a wider area on the C-S-H surface thereby enhancing the hydrophobic effect. In summary, the utilization of ETES presents notable advantages within the context of low Ca/Si ratio cement. This study contributes valuable insights into the design and application of silane coatings. • Assessment of ETES chemical reactivity on C-S-H surface. • Reveal the effect of C/S ratio on ETES adsorption mechanism. • Evaluate the breaking energy barrier of Si CSH -O-Si ETES linkage. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dehydration and dehydroxylation successively occurs as hydration degree decreases. • Dehydroxylation leads to interlayer spacing collapse and disordering primary layers. • Interlayer spacing collapse significantly stiffens and toughens the C-S-H. • Drastic volume shrinkage decreases the packing density of C-S-H gel. The dehydration and dehydroxylation of C-S-H gel ubiquitously occurs in the thermal destruction or recycle of the concrete materials, and the latter is closely related to the sustainability in the construction industry. Utilizing reactive molecular dynamics simulation, this paper presents an investigation on the structure, dynamics and mechanical properties evolution of the C-S-H with decreasing water content, at the molecular level. Dynamically, it is found that the dehydration and dehydroxylation are successively occurred as the hydration degree of C-S-H decreases. At the dehydration stage, water molecules are dissociated to maintain the number of hydroxyls. The C-S-H structure does not change much at this stage. At the dehydroxylation stage, the number of hydroxyl groups sharply decreases, along with large structural transformation in the C-S-H, including fast decreasing interlayer spacing, merging of interlayer calcium layers and disordering of the primary calcium silicate layers. With respect to the mechanical properties, the interlayer spacing collapse significantly increases the stiffness and toughness of the C-S-H structure. On the other hand, drastic volume shrinkage of the C-S-H structure due to dehydroxylation can lead to decreasing contact points between C-S-H nano-globules. This means an increase in the mechanical properties of C-S-H matrix but a decrease in the packing density of C-S-H gel. Eventually, the indentation modulus of C-S-H gel monotonously decreases during dehydration and dehydroxylation. [ABSTRACT FROM AUTHOR]

In this study, a novel graphene oxide (GO) and epoxy (EP) composite coating was proposed to prevent the ingress of the water and ions in the nanometer channel of calcium silicate hydrate (C–S–H) gel. To evaluate the water resisting properties of the GO-EP composite coating, the local structure, dynamic properties and water transport mechanism have been studied by molecular dynamic method. As compared with single coating with graphene oxide, the migration of water and ions is significantly inhibited in the C–S–H gel pore impregnated with GO and hydrophobic epoxy composite. The GO plays critical role in bridging the C–S–H gel and epoxy composite. The functional groups in the GO sheets associate with the interfacial calcium atoms in the interior surface of C–S–H gel and provide oxygen sites to accept the H-bonds from the silicate tetrahedron. On the other hand, the H-bonds donated by the hydroxyl in the GO sheets to epoxy help strengthen the connection between the GO sheets and epoxy molecules. During the capillary transport process, the water molecules are firstly dragged by the hydroxyl groups on the GO sheets, accept or donate their H-bonds to functional groups and diffuse in the vacancy region between epoxy molecules and GO sheet. The hydrophobic epoxy molecules, blocking the C–S–H gel pore, inhibit the solution invasion. With the coating percentage of epoxy molecules from 21.8% to 87.2%, the gyration radium of molecule increase from 10.42 to 10.52 and the penetration depth of water molecules is dramatically slowed down by 71.58%. While the penetrated solution results in the reduction of adhesive energy by 66.38% between GO sheets and epoxy group, the adhesive energy between GO and C–S–H remains constant. The incorporation of GO sheet between C–S–H and epoxy can contribute to the stability of the composite coating. Furthermore, the mobility of Na and Cl ions is reduced due to the presence of the GO-EP composite coating, which immobilizes the penetrated Na and Cl ions in the entrance region and separates the ions from the water solution. Hopefully, the novel hydrophilc-hydrophobic composite coating provides valuable insights on the design of waterproof material. [ABSTRACT FROM AUTHOR]

• Epoxy detaches gradually from the C-S-H surface under aqueous and saline conditions. • The local structure of water molecules and epoxy on the C-S-H substrate were depicted. • Water molecules play an important role in the configurational changes of epoxy molecules. Reliable bonding between epoxy and concrete is necessary for FRP bonded concrete structures to ensure their durability. However, FRP bonded concrete structures are inevitably exposed to aqueous or saline environments, which would extremely threaten the interfacial bonding of structures. Here, the bonding properties of epoxy/hydrated calcium silicate (C-S-H) interface under aggressive environments is investigated by using molecular dynamics (MD) simulation. The configurational analysis reveals that the epoxy detaches gradually from the C-S-H surface under aqueous and saline conditions, indicating that the deterioration of the bonding between epoxy and C-S-H matrix. Simulation results show that the Ca w -O C-S-H and Ca w -O epoxy connections are essential for good bonding of the epoxy/C-S-H interface. The H-bonds, formed between the epoxy and C-S-H matrix, are also not negligible for improving the interfacial bonding. In an aggressive environment, water molecules are adsorbed on the surface and become the main factor in the configurational changes. The Ca w -O C-S-H , Ca w -O epoxy connection, and the H-bonds network will be weakened or destroyed under the effect of water molecules. This study could provide valuable information on the understanding the bonding degradation and configurational changes between epoxy and C-S-H matrix in different environments. [ABSTRACT FROM AUTHOR]

This work investigates the interfacial bonding properties between calcium silicate hydrate and fibers at the micro-level. • The interfacial shear strength between C-S-H and polymer fibers is closely related to the fiber types. • Ca atoms in the interface play an important role in the interfacial bonding interactions. • H bonds provide contribution for enhancing the interfacial bonding strength. The macroscopic mechanical performance of fiber reinforced concrete (FRC) is determined by the interfacial microstructure and the interfacial bonding properties between hydration products and fibers. This research is dedicated to study the interfacial shear strength, structure and dynamics between calcium-silicate-hydrate (C-S-H) and polymer fibers. To better understand the effects of fiber types on the interfacial shear strength, three kinds of fibers were investigated, including polypropylene (PP), polyvinyl alcohol (PVA) and polyacrylic acid (PAA). The calculation results indicate that the interfacial shear strength is closely related to the fiber types. The interfacial shear strength of fibers restricted in the C-S-H substrate was arranged in the following order PAA fiber > PVA fiber > PP fiber. After measuring the interfacial shear strength, the static molecular structure and dynamics properties, especially at the interface between the fiber and the C-S-H matrix, the cause for the interfacial shear strength discrepancy between different fibers was uncovered. It shown that the Ca atoms in the interface play an important role in the interfacial bonding interactions by the formation of O CSH -Ca-O polymer connection. The H bonds can be formed between PAA/PVA fibers and C-S-H substrate, also provide some contribution for enhancing the interfacial bonding strength. The chemical bond stability revealed that the interfacial bonding strength of three fibers follows the order of PAA fiber > PVA fiber > PP fiber, which is consistent with the order of interfacial shear strength. [ABSTRACT FROM AUTHOR]

Ductility of fiber reinforced cement-based materials is closely related to the interfacial bond of fiber/matrix, especially for the fiber with surface chemical inertness. In this study, graphene oxide (GO) was coated on polyethylene (PE) fiber to strengthen its bond to matrix. Firstly, the effect of PE fiber with and without GO coating on the microstructure and tensile properties of cement mortar was investigated. Compared with PE-Mortar, GO/PE-Mortar showed a significant enhancement in first cracking strength, ultimate tensile strength and strain by 11.7%, 46.3% and 70.4%, respectively. The hydrogen at honeycomb lattice of GO and oxygen in the hydroxyl and epoxy of GO can interact with oxygen in calcium silicate hydrogen (C–S–H) and hydrogen in PE chains via intermolecular H-bonding, contributing to strengthen the bond of PE fiber/matrix. Secondly, the molecular dynamics (MD) simulation results revealed that the atomic translational motion across the interface of C–S–H/PE was decreased after the coating of GO, and the whole system was thus becoming more stable. Moreover, in the pull-out simulation, the maximum force for PE fiber to be pulled out from the C–S–H structure was increased by 41.67%, because GO helped to turn the weak C–S–H/PE interface to be that with a higher interfacial bonding energy at the C–S–H/GO and GO/PE interfaces. In conclusion, GO, as a promising coupling material, has great potential to strengthen the interfacial bond between the PE fiber and cement matrix, to develop cement-based materials with enhanced ductile properties. [ABSTRACT FROM AUTHOR]

The chloride transport through gel pores of cementitious matrixes determines the durability and sustainability of concrete, the most widely applied construction materials. In this work, a smart polymer is designed by molecular dynamics, which can effectively inhibit the transport of fluids throughout the nano-channels of calcium silicate hydrate (C–S–H). This polymer chain owns a high-polarity carboxyl group at one end, which can adsorb stably on the C–S–H surface by forming high-strength interfacial connections, while the rest parts are all hydrophobic alkyl groups. The polymer chain acts like a unilateral gate, which is open (lie on the matrix surface) when located in the dry nano-pore of C–S–H. Nevertheless, it can be closed rapidly (stand upright, vertical to the matrix), utilizing the hydrophobic groups to maximize the inhibiting effect once upon contact with the advancing fluids. Furthermore, a novel polymer-based nano-material is fabricated in the light of this philosophy, and the performance evaluation experiments are implemented. The results prove this material substantially decreases the water adsorption amount and chloride migration rate of the cement-based materials. Image 1 • A special chemical structure of smart polymers is designed to realize the intelligent fluid transport. • Smart polymer acts like a unilateral gate at the nano-channels of the porous matrix. • The gate is getting closed once upon the contact with fluids. • Strong interfacial connections form between smart polymers and the matrix. • The hydrophobic groups of smart polymers inhibit the transport of fluids. [ABSTRACT FROM AUTHOR]